Method for producing polyamide resin film

ABSTRACT

[Problem] To provide a method for producing a polyamide resin film by using a poi amide resin obtained through polymerization of a regenerated monomer used as a recycled material. [Solution] Provided is a method for producing a polyamide resin film, including: a step of producing a monomer from a raw material (A) for depolymerization, (2) a step of producing a polyamide resin (B) through polymerization using a raw material containing the monomer, (3) a step of refining the polyamide resin (B), and (4) a step of producing an unstretched film using a starting material containing the refined polyamide resin (B), and stretching the unstretched film.

TECHNICAL FIELD

This invention relates to a method for producing a polyamide resin filmcontaining a polyamide resin made from a large amount of recycled rawmaterial.

BACKGROUND ART

It has been a long time since environmental issues began to attractattention. The awareness of the need to ensure environmentalsustainability has heightened in recent years once again. There has beena high demand for recycling resin waste materials from before, andrecycled resins from resin waste materials have ween used as rawmaterial resins. Examples of waste materials produced during theproduction of thermoplastic resin stretched film include waste scrapssuch as edge trimming scraps and slit scraps, as well as films that werenot commercialized as defective products and the like. Materialrecycling has commonly conducted using pellets made by melting thesewaste materials again as remelted resins (raw materials).

However, material recycling reduces productivity, especially forpolyamide resin films with high stretching stress, because the contentof foreign matters, thermal degradation products, and the like increaseswith the increase in the content of remelted resin, which makes the filmmore prone to be cut during stretching. Furthermore, it is difficult tocontrol the amount or additives contained in the remelted resin, andtherefore, local characteristic changes may occur on the film surface,which may result in printing defects, adhesion failures, and otherproblems. Therefore, the amount remelted resins that can be contained ina polyamide resin film is limited, and it is considered that there arealso limits to increasing recycling rates.

Meanwhile, laminated films, such as packaging films, in which multipletypes of resin materials, including a printing layer, a barrier layer,an adhesive layer, and a sealant layer, are laminated, are difficult toseparate into each raw material, and sometimes difficult to recycle.

In such a case, technique of depolymerizing resin waste materials andusing the depolymerized product as monomers (regenerated monomers) hasbeen known, and such a technique is also called chemical recycling. Inparticular, various chemical recycling methods have also been proposedfor polyamide resins. For example, a method has been known, includingmelting nylon 6, removing metals, recovering ε-caprolactam obtained bydepolymerization reaction to recycle the recover ε-caprolactam as afiber aw material, resin raw material, and the like (see PTL 1 and PTL2).

CITATION LIST Patent Literature

[PTL 1] Japanese Patent Application Publication No. H07-330718

[PTL 2] Chinese Patent Application Publication No. 91106147.9

SUMMARY OF INVENTION Technical Problem

However, even if the monomers obtained by depolymerization of a resinwaste material of a polyamide resin are polymerized, polyamide resinssuitable for films cannot be obtained. That is, a film having excellentstrength, elasticity, and the like cannot be obtained.

In addition, in recent years various printings are made on productpackages, such as packaging containers, and demands for an appearanceafter printing tend to be increasingly higher. Meanwhile, there is roomfor improving the suitability for printing on films (in particular,reproducibility of light color portions (low-gradation portions) in adeep and light color expression).

Accordingly, the main object of the present invention is to provide amethod for producing a polyamide resin film by using a polyamide resinobtained through polymerization of a regenerated monomer used as arecycled

Furthermore, the present invention has an object to provide a polyamideresin film that can exhibit printability equivalent to or better thanconventional products.

Solution to Problem

The present inventor has made intensive studies in view of the problemof the conventional art, and as a result, has found that the objectsmentioned above can be achieved by using a specific raw materialcontaining a regenerated monomer in the production of films, andcompleted the present invention.

That is, the present invention relates to the following polyamide resinand production method thereof.

1. A method for producing a polyamide resin film, the method including:

-   -   (1) a step of producing a monomer from a raw material(A) for        depolymerization;    -   (2) a step of producing a polyamide resin (B) through        polymerization using a raw material containing the monomer;    -   (3) a step of refining the polyamide resin (B); and    -   (4) a step of producing an unstretched film using a starting        material containing the refined polyamide resin (B), and        stretching the unstretched film.

2. The production method according to 1 above, wherein the monomercontains ε-caprolactam.

3. The production method according to 1 above, further including a stepof adjusting moisture content in an unstretched film to 2 to 10 mass %in advance, prior to stretching.

4. The production method according to 1 above, wherein startingmaterials partly contain a remelted resin obtained by melting apolyamide resin (provided that the polyamide resin (B) is excluded) anda content of the remelted resin is 1 mass % or more.

5. The production method according to 1 above, wherein the refinedpolyamide resin (B) has a relative viscosity ηR of 2.5 to 4.5.

6. The production method according to 1 above, wherein the raw material(A) for depolymerization is at least one of a polyamide 6 resin and asoligomer thereof.

7. A polyamide resin film, wherein, when the film is halftone-printed,the polyamide resin film includes 100 or less missing dots in 1000 mm²of a 10% gradation part in a halftone-printed film.

8. The polyamide resin film according to 7 above, which has acaprolactam monomer concentration of 1.6 mass % or less in the polyamideresin film.

9. The polyamide resin film according to 7 above, wherein an aminoterminal group content and a carboxyl terminal group content in thepolyamide resin film are each 80 mmol/kg or less.

10. The polyamide resin film according to 7 above, which has a ratio(minimum/maximum) of 0.5 to 1.0 between a maximum and a minimum of filmimpact strengths under an atmosphere at a temperature of 23° C. and ahumidity of 50% RH.

11. The polyamide resin film according to 7 above, which includes apolyamide resin that shows a half width of 10° C. or more of acrystallization peak during cooling.

12. The polyamide resin film according to claim 7, is biaxiallyoriented.

13. A laminated film including the polyamide resin film according to anyone of 7 to 12 above, and a sealant resin layer laminated on thepolyamide resin film.

14. A packaging material including the polyamide resin film according toany one of 7 to 12 above.

15. A polyamide resin film obtained b the method according to any one ofto 6 above.

Advantageous Effects of Invention

The present invention can provide a polyamide resin film that canexhibit a printability equivalent to or better than conventionalproducts (brand-new products) even when a polyamide resin obtainedthrough polymerization of a regenerated monomer used as a recycledmaterial. More specifically, the present invention can provide apolyamide resin film (particularly, a biaxial-stretched polyamide resinfilm) that efficiently inhibits deterioration of local printingappearance, deterioration of adhesion properties, and the like by usinga polyamide resin made from a regenerated monomer.

Moreover, the polyamide resin film of the present invention can besuitably used, especially as a packing material, because of its reducedimpact strength unevenness and excellent general properties such as filmelasticity, wettability, and haze.

Since the production method of the present invention allows the use of aresin waste material as a raw material, the production method canprovide a recycled polyamide resin film with a high recycle ratio of,for example, 50% or more. This promotes the reuse of resources, whichmakes it possible to contribute to environmental preservation as asustainable technique.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating the outline of the method forevaluating the printability of polyamide resin film of the presentinvention,

DESCRIPTION OF EMBODIMENTS 1. Production Method of Polyamide Resin Film

The production method of the present invention is a method for producinga polyamide resin film, the method including:

-   -   (1) a step of producing a monomer from a raw material (A) for        depolymerization (depolymerization step);    -   (2) a step of producing a polyamide resin (B) through        polymerization using a raw material containing the monomer        (polymerization step);    -   (3) a step refining the polyamide resin (B) (refining step); and    -   (4) a step of producing an unstretched film using a starting        material containing the refined polyamide resin and stretching        the unstretched film (filming step).

Depolymerization Step

In the depolymerization step, a monomer is regenerated from a rawmaterial for (A) for depolymerization (hereinafter, such a monomer isreferred to as a “regenerated monomer”).

The regenerated monomer is particularly preferably a lactam, and forexample, ε-caprolactam, enantolactam, capryllactam, lauryl lactam, andthe like can be used. Among them, ε-caprolactam is particularlypreferred.

The type of the raw material (A) for depolymerization is notparticularly limited, and various polyamide resins, as well as oligomersof various polyamide resins can be used. More specifically, variousresins listed as the polyamide resin (B), which will be mentioned later,can be mentioned as examples thereof. Examples of the oligomer mayinclude chain substances from dimers up to about heptamers and circularsubstances from dimers up to about nonamers, and the like.

In particular, at least one of polyamide resins and oligomers thereofcan suitable be used as the raw material (A) for depolymerization in thepresent invention. In particular, polyamide 6 is a resin substantiallyonly constituted of ε-caprolactam as the monomer units and, therefore,has an advantage in that depolymerization to monomers and purificationand separation are easy.

Examples of the form or the polyamide resin include discharged resinscraps, including the portion during the switching between grades inpolymerization and the portion during the switching before the start ofobtaining film products; film waste scraps, including edge trimmingscraps and slit scraps that are generated during film production; filmsthat were not commercialized as defective products, and the like. Usingthese materials as raw materials makes it possible to contribute to theproduction of films with excellent printability and the like, and alsoto contribute to environmental preservation through the use of wastematerials.

Examples of the oligomer form include oligomers having high watersolubility which are recovered from used water in refining of apolyamide resin, residues after the filtration of the water whichcontain dimer to octamer having low water solubility, and the like.

The method for producing a monomer from the raw material (A) fordepolymerization is not particularly limited as long as thepredetermined monomer can be obtained, and a depolymerization reactionof the raw material (A) for depolymerization can be preferably employed.That is, a regenerated monomer can be suitably obtained by chemicallydecomposing the raw material for depolymerization through adepolymerization reaction.

The method and condition of the depolymerization reaction are notparticularly limited, and the depolymerization reaction can be conductedaccording to a known method. Accordingly, for example, a catalyst may ormay not be used. Furthermore, the depolymerization reaction may beconducted in the absence of water (dry method) or the presence of water(wet method). Especially from the viewpoint of productivity, a methodconducting depolymerization in hot steam under the presence of acatalyst is preferred. A less water-soluble cyclic oligomer is hard todepolymerize directly because the hydrolysis rate of amide bonds isslow. In contrast, ring-opening polymerization of a cyclic oligomer intochain molecules and subsequent depolymerization under the conditionmentioned above allows for suitable production of regenerated monomerseven from a cyclic oligomer.

Polymerization Step

In the polymerization step, a polyamide resin (B) is produced bypolymerization using a raw material containing the monomer mentionedabove (the regenerated monomer).

The raw material may be a raw material in which all monomers therein areregenerated monomers, but a raw material combinedly containing virginmonomers together with regenerated monomers is preferred. A virginmonomer herein is an antonym of a regenerated monomer and refers to amonomer that has not undergone a depolymerization step of polymers. Thevirgin monomer may be a commercially available product. For example, amonomer normally available on the market can be used as a virginmonomer.

Regenerated monomers may contain hardly-separatable byproducts. This canslightly slow down the rate of crystallization of the polyamide resinmade only from a regenerated monomer compared to a polyamide resin madeonly from a virgin monomer, which allows for the further slowdown of therate of crystallization of a polyamide resin made from a combination ofa regenerated monomer and virgin monomer. A half width is used as anindex of the crystallization rate. The half width is determined bymeasuring the crystallization temperature during cooling (T_(C)) of theresulting polyamide resin. The half width in the present invention isnormally preferably 10° C. or higher, particularly preferably 11° C. orhigher, and among them, most preferably 12° C. or higher. The wider thehalf width, the wider the rate of crystallization, the less likely thefilm is to have localized irregularities in ne crystalline state on thefilm surface when stretched and crystallized, and the more uniform thefilm is, which improves the printability of low gradation portions. Fromsuch a viewpoint, it is preferred to use a polyamide resin polymerizedfrom a combination of a regenerated monomer and a virgin monomer. Itshould be noted that the upper limit of the half width can be set to,for example, about 20° C., but is not limited thereto.

The regenerated monomer content in the raw material is not particularlylimited, and the upper limit thereof is preferably set to 90 mass % orless and more preferably 80 mass % or less in view of broadening thehalf width and the like. The lower limit thereof is not particularlylimited and is preferably set to 5 mass % or more, particularlypreferably 10 mass % or more in view of increasing the recycling rate.

A virgin monomer is preferably used in combination as a component otherthan the regenerated monomer. In this case, the virgin monomer contentin the raw material is normally preferably about 10 to 95 mass % andparticularly preferably 20 to 90 mass %.

For example, ε-caprolactam regenerated by depolymerization reaction of apolyimide 6 resin (hereinafter expressed as “C-CL”) may be used within arange close to 100 mass %. Meanwhile ε-caprolactam as a virgin monomer(hereinafter expressed as “V-CL”), may be contained as a monomer otherthan C-CL.

In addition, the terminals of the polyamide resin (B) may be optionallycapped for the purpose of inhibiting the monomer production duringmelting. For this reason, the raw material may optionally contain anadditive such as a terminal blocking agent. The terminal blocking agentis not particularly limited, and for example, may be an organic glycidylester, a dicarboxylic anhydride, a ono carboxylic acid such as benzoicacid, a diamine, and the like.

Polymerization methods themselves for producing a polyamide resin (B)are not particularly limited, and any known method for polymerizing amonomer can be employed. An example that can be employed as the methodis a method including mixing ε-caprolactam, water, and benzoic acid as aterminal blocking agent; performing the polymerization reaction whilethe mixture is heated, pressurized, depressurized, and dehydrated in apolymerization vessel until the viscosity reaches the target value.

Refining Step

In the refining step, the polyamide resin (B) is refined. This allowsmonomers contained in the polyamide resin to be removed and the relativeviscosity of the polyamide resin to be increased to the desired range,which results in properties suitable for film formation.

The refining method is not limited, but the refining is preferablyconducted such that the relative viscosity (25° C.) of the polyamideresin (B) is particularly within the range of about 2.5 to 4.Accordingly, it is preferred to refine the polyamide resin (B) for 15 to30 hours using hot water at 90° C. to 100° C., for example.

As refining methods themselves, any known method, such as a method forimmersing the polyamide resin (B) in hot water, may be employed. In thiscase, the polyamide resin (B) may be refined, for example, in the formof a molded body such as pellets.

The refining step may be conducted as a single treatment step, and mayoptionally be conducted as two or more treatment steps. Sufficientrefining allows the monomers contained in a polyamide resin to be elutedmore completely into hot water, thus sufficiently lowering the monomercontent in the refined polyamide resin.

A polyamide resin after the refining step is preferably dried accordingto need. The drying condition is not particularly limited. For example,hot-air drying can be conducted at a temperature of about 100° C. to130° C. for 10 to 30 hours, but is not limited thereto. Morespecifically, hot-air drying at 110° C. for 20 hours can be conducted.

Filming Step

In the filming step, an unstretched film is prepared from a startingmaterial containing the refined polyamide resin (B) and then stretched.

The starting material may contain a polyamide resin (B) but mayoptionally contain other components. For example, a remelted resin (D)obtained by melting a polyamide resin (provided that the polyamide resin(B) is excluded) within a range that does not adversely affect theadvantages of the present invention in order to increase the cost merit,recycling rate, and the like. In the present invention, resin wastes ofa polyamide resin (in particular, a polyamide 6 resin) may be preferablyused as the remelted resin (D) from the viewpoint of environmentalpreservation, effective use of resources, and the like. Morespecifically, unstretched scraps, edge trimming scraps, slit scraps thatare produced during the production of a polyamide resin film, pelletsformed by re-melting defective products and the like may be used.

The content of the remelted resin (D) in the starting material is notparticular limited and is preferably set to 1 mass % or more andparticularly preferably 5 mass % or more for increasing the recyclingrate. Meanwhile, the upper limit of the content of the remelted resin(D) is normally 75 mass % or less, particularly preferably 65 mass % orless, further preferably 50 mass % or less, still more preferably 40mass % or less, and among them, most preferably 35 mass % or less. Theremelted resin tends to show an increased an amount of terminal inmolecular chain, decreased viscosity, and poor quality stability.Therefore, if the content exceeds 75 mass %, the relative viscosity as afilm raw material decreases, and foreign substances, thermally degradedsubstances or the like increase, which tends to cause troubles such asbreaking during film formation. In addition, the mechanical propertiesof a film tend to deteriorate, such as the decrease in tensileelongation. Moreover, the content of caprolactam monomers and oligomerssuch as dimers, which are generated in the stretching step, increases,which may cause problems, such as localized deterioration inprintability in the subsequent steps and increase of the bag breakagerate due to poor adhesiveness. Therefore, the influence on practicalperformance is concerned.

Film scraps, defective products, and the like used as remelted resinsnormally contain additives such as lubricants, oxidation preventionagents, and the like, and the content mentioned above shall be a contentcontaining these additives.

The concentration of additives such as lubricants, oxidation preventionagents in film scraps, defective products, and the like varies dependingon grades or the like. Therefore, as the content of the remelted resinin the polyimide resin film is higher, the haze, the wetting tension ofthe film surface, suitability for printing, adhesion properties, and thelike of the resulting film may locally be lowered due to variations inthe concentration distribution of the additives. For this reason, whenthe polyamide resin film is composed or a plurality of layers, theremelted resin content in the front surface layer (the outermost layer)is preferably 50 mass % or less, particularly preferably 40 mass % orless, and among them, most preferably 35 mass % or less.

Meanwhile, in an intermediate layer (a layer other than the outermostlayer), the remelted resin content can be increased to increase therecycling rate because the characteristics on the film surface, such aswetting tension or printability, are not affected even if the remeltedresin content is large. However, the remelted resin content in anintermediate layer is preferably 75 mass % or less because the film hazemay increase, and the physical properties such as impact strength tendto deteriorate. The lower limit of the remelted resin content in eachlayer can be set to, for example, about 5 mass %, but is not limitedthereto.

As other components contained in the starting material, a polyamideresin (C) obtained by polymerizing a raw material containing noregenerated monomer but only consisting of virgin monomers may becontained.

When a mixture of a polyamide resin (B) polymerized only fromregenerated monomers and a polyamide resin (C) polymerized only fromvirgin monomers as another resin is used as starting materials and themixture is melt-extruded, physical properties close to a polyamide resin(B) obtained by polymerizing a combination of regenerated monomers andvirgin monomers can be obtained. Meanwhile, from the viewpoint ofobtaining more uniform film surface characteristics, it is preferred topolymerize a combination of regenerated monomers and virgin monomers.That is, a polyamide resin (B) constituted of a polymer containing aregenerated monomer and a virgin monomer as monomer units can suitablybe used.

The relative viscosity of each polyamide resin contained in the startingmaterial is not limited, but is particularly preferably 2.5 to 4.5, andamong them, more preferably within the range of 2.8 to 4.0. If apolyamide resin with a relative viscosity of less than 2.5, filmformation and stretching are difficult, and even if a polyamide resinfilm is obtained, the mechanical characteristics may be significantlylowered. In contrast, film formability may be hindered when a polyamideresin with relative viscosity over 4.5 is used.

The relative viscosity in the present invention a value of a samplesolution (liquid temperature: 25° C.), prepared by dissolving a resin tobe measured in 96% sulfuric acid such that the concentration should be1.0 g/dl, measured by using an Ubbelohde viscosimeter.

Furthermore, the starting material may optionally include one of, or twoor more of various additives such as oxidation prevention agents, UV rayabsorbing agents, preservatives, antistatic agents, blocking preventionagents, and inorganic fine particles within a range that does notadversely affect the performance of the film.

Furthermore, a lubricant may be mixed in the starting material for thepurpose of improving the slip properties of an obtained film and thelike. Both inorganic and organic lubricants can be used as thelubricant. Specific examples of lubricants include clay, talc, calciumcarbonate, zinc carbonate, wollastonite, silica, alumina, magnesiumoxide, calcium silicate, sodium aluminate, calcium aluminate, magnesiumaluminosilicate, glass balloon, carbon black, zinc oxide, antimonytrioxide, zeolite, hydrotalcite, layered silicate, ethylenebis-stearamide, and the like. Among them, silica is particularlypreferred. The lubricant content is not particularly limited, and therange from 0.01 to 0.3 mass % in the starting material is appropriate.

The method for manufacturing an unstretched film is not limited and maybe formed by a known film formation method. For example, an unstretchedfilm may be prepared by extruding the melted material of the rawmaterial from a T-die and cooled on a casting roll. In this case, it isrequired to control the actual temperature on the casting roll surfacewith high accuracy from the viewpoint of making the degree ofcrystallinity of the unstretched film uniform.

In the present invention, if necessary, it is preferred to control themoisture content in an unstretched film to about 2 to 10 mass % (inparticular, 4 to 8 mass %) as a moisture control step, and then stretchthe unstretched film. If the moisture content is lower than 2 mass %,stretching stress may increase and troubles, such as film breakage, arelikely to occur. If the moisture content is higher than 10 mass %, theunevenness of the thicknesses in an unstretched film may be large, andthe variation of the thicknesses of the resulting stretched film is alsolarge.

In the moisture control step, in general, when the moisture content of afilm is low (especially less than 2 mass %), the moisture content of thefilm is controlled by passing the film through a moisture control tankat a temperature of 40° C. to 90° C., further preferably 50° C. to 80°C., and adjusting the passing time. Pure water is normally used in themoisture control tank, but the treatment liquid may optionally contain adye, a surfactant, a plasticizer, and the like. Alternatively, themoisture may be controlled by spraying water vapor. On the other hand,if the moisture content of a film is high (especially more than 10 mass%), the moisture content of the film may be controlled by passing thefilm through a drying furnace and adjusting the passing time.

Next, the unstretched film is stretched. The way of stretching theunstretched film is not particularly limited, and any stretching methodcan be employed. In particular, it is preferred to perform biaxialstretching from the viewpoint of preparing a film with excellent tensileelongations both in MD and ID directions.

The biaxial stretching method is not limited, and for example, asimultaneous biaxial stretching method and a sequential biaxialstretching method may be mentioned. For example, the simultaneousbiaxial stretching, method is preferred in terms of surface balance toreduce the differences between the MD and ID directions in propertiessuch as tensile elongation, elastic modulus, and tensile strength. Incontrast, the sequential biaxial stretching method is preferred forimproving puncture strength, impact strength, and the like. According tothe desired film properties, uses, and the like, these stretchingmethods may be selected as appropriate.

The draw ratio can be set, as appropriate, according to the use, desiredproperties, and the like. For example, the draw ratio can be set to 2 to4 times in the MD direction and 2 to 4 times in the TD direction,although it is not limited thereto. Also, the stretching temperature isnot limited, and the stretching cab be conducted, for example, at atemperature within the range of 40° C. to 220° C. In particular, in thecase of sequential stretching, the stretching in the MD direction ispreferably conducted at a temperature of 40° C. to 80° C., and thestretching in the TD direction is preferably conducted at a temperatureof 80° C. to 150° C. In the case of simultaneous biaxial stretching, thetemperature is preferably set to 160° C. to 20° C.

The resulting biaxially-stretched film is preferably subjected toshort-time heat treatment at about 150° C. to 220° C. according to needin order to improve dimension stability and reduce hot water shrinkage.

Furthermore, the polyamide resin film of the present invention may besubjected to surface treatment such as corona discharge treatment andadhesiveness-increasing treatment in a range that does not substantiallyadversely affect the advantages of the present invention. For example, ahighly-adhesive laver, a harrier coat layer, a printing layer, and thelike can be disposed as appropriate.

The polyamide resin film of the present invention obtained in this waycan optionally be provided as a laminated body laminated with anotherlayer.

Furthermore, a film comprising a plurality of layers may be formed bysimultaneous melt extrusion, lamination, of the like in themanufacturing stage of the polyamide resin film. For example, thepolyamide resin film may have a two-type two-layer constitution, inwhich a polyamide resin film containing a polyimide 6 resin obtained bypolymerizing a regenerated monomer and a polyamide 6 resin filmcontaining a remelted resin, a two-type three-layer constitution, inwhich polyamide resin films containing a polyamide 6 resin film obtainedby polymerizing a regenerated monomer sandwich a polyamide 6 resin filmcontaining a remelted resin, and the like. A polyamide 6 resin filmhaving a constitution that can sandwich an intermediate layer, such asthe two-type three-layer constitution, can increase the remelted resincontent in the intermediate layer.

2. Polyimide Resin Film

The present invention includes a polyamide resin film, wherein when thefilm is halftone-printed, the halftone-printed film includes 100 orfewer missing dots in 1000 mm² of a 10% gradation part in ahalftone-printed film.

(1) Composition of Polyamide Resin Film

The composition of the polyamide resin is not particularly limited; forexample, a polyamide 6 resin made from lactams such as ε-caprolactam maybe mentioned. Examples of other polyamide resins include polyamideresins obtained by polycondensing lactams with 3 or more membered ring,o-amino acids, dibasic acids, diamines, and the like.

More specifically, examples of the lactams include ε-caprolactammentioned above, and enantolactam, capryllactam, lauryl lactam, and thelike.

Examples of the ω-amino acids include 6-aminocaproic acid,7-aminoheptanoic acid, 9-aminonbnanoic acid, 11-aminoundecanoic acid,and the like.

Examples of the dibasic acids include adipic acid, glutaric acid,pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioicacid, dodecanedioic acid, hexadecanedioic acid, eicosanedioic acid,eicosadienedioic acid, 2,2,4-trimethyladipic acid, terephthalc acid,isophthalic acid, 2, 6-naphthalenedicarboxylic acid, xylylenedicarboxylic acid, and the like.

Examples of diamines include ethylene diamine, trimethylene diamine,tetramethylene diamine, hexamethylene diamine, pentamethylene diamine,undecamethylene diamine, 2,2,4 (or 2,4,4)-trimethylhexamethylenediamine, cyclohexanediamine, bis-(4,4′-aminocyclohexyl)methane,m-xylylene diamine, nonane diamine, decane diamine, and the like.

Examples of the polymers and copolymers that can be obtained bypolycondensing these monomers include polyamides 6, 7, 10, 11, 12, 4.10,5.6, 6.6, 6.9, 6.10, 6.11, 6.12, 10.10, 6T, 91, 10T, 61, MXD6(m-xylylene diadipamide 6), 6/6.6, 6/12, 6/6T, 6/61, 6/1=6, and thelike. Among them, polyamide 6 resins are preferred in the present.invention in view of the fact that the balance between heat resistanceand mechanical characteristics is excellent. That is, the polyamideresin film of the present invention is preferably a polyamide 6 resinfilm.

(2) Properties of Polyimide Resin Film

The polyamide resin film of the present invention has a printabilitythat, when the film is halftone-printed, the halftone-printed filmincludes 100 or fewer missing dots in 1000 mm² of a 10% gradation partin the halftone-printed film. A film having such characteristics isadvantageous upon printing because of the appearance after printing andhigh reproducibility, especially in a light color portion among deep andlight color portions. The number of the missing dots is preferably 80dots or fewer, more preferably 70 dots or fewer, and among them, mostpreferably 60 dots or fewer. If the above number exceeds 100 dots, theirregularities are visually observed even in low-gradation portions bygazing at the area, and individual differences in the pattern can befelt, which may be judged to be a printing defect. The lower limit ofthe above number is most preferably 0. For example, the number ofmissing dots may be about 30, although it is not limited thereto.

The polyamide resin film of the present invention has a relativeviscosity ηR of preferably within. the range of 2.5 to 4.5 and morepreferably within the range of 2.8 to 4.0. The relative viscosity in thepresent invention is a value of a sample solution (liquid temperature:25 °C.), prepared by dissolving a film to be measured in 96% sulfuricacid such that the concentration should be 1.0 g/dl, measured by usingan Ubbelohde viscosimeter. A polyamide resin film having a relativeviscosity of less than 2.5 may show significantly deteriorated dynamiccharacteristics, or the evaluation thereof in the bag drop testregarding the laminated film, which will be mentioned later, may belowered. In addition, a polyamide resin film having a relative viscosityexceeding 4.5 is likely to show insufficient film thickness uniformityand may cause irregularities in mechanical characteristics.

The accuracy in thickness of the polyamide resin film of the presentinvention is preferably within the range of plus or minus 15% withrespect to the target thickness and particularly preferably within therange of plus or minus 10% with respect to the target thickness.

The extracted amount of caprolactam monomers from. the polyamide resinfilm of the present invention is preferably 1.6 mass % or less, morepreferably 1.0 mass % or less, still more preferably 0.5 mass % or less,and among them, most preferably 0.1 mass % or less. If the extractedvolume of caprolactam monomers from a film exceeds 1.6 mass %, thewettability of polyamide resin film surface is more likely to vary.Furthermore, the adhesive performance between the polyamide resin filmand another resin layer or ink may be locally lowered, resulting indeterioration of practical performances, for example, a decrease innumber up to breakage of bags in a bag drop test.

The polyamide resin film of the present invention has a terminal aminogroup content and a terminal carboxyl group content in the polyamideresin film of preferably 80 mmol/kg or less, most preferably 70 mmol/kgor less, respectively. If either of the terminal amino group andterminal carboxyl group content exceeds 80 mmol/kg, the crystallizationrate of the film may be increased, and the local unevenness in thedegree of crystallinity are more likely to occur. This may decrease thelocal printability of the film and is likely to decrease the tensileelongation and cause unevenness in properties such as impact strength,and even may partially decrease the properties.

The polyamide resin film of the present invention shows a ratio(minimum/maximum) between a maximum and a minimum of film impactstrengths under an atmosphere at a temperature of 23° C. and a humidityof 50% RH of preferably 0.5 to 1.0, particularly preferably 0.6 to 1.0,and among them, most preferably 0.7 to 1.0. It is considered that thereis a partial reduction in the impact resistance of the film when theabove ratio is less than 0.5, which is not desirable from a practicalstandpoint.

The tensile elongations in both MD and ID directions are preferably 60%or more and particularly preferably 70% or more.

The polyamide resin film of the present invention preferably has a hazevalue of 10.0% or less, particularly 7.0% less, and among them, mostpreferably 6.0% or less.

The polyamide resin film of the present invention is preferably filmobtained by the “1. Production Method of Polyamide Resin Film”. That is,a film produced using a raw material including a regenerated monomerpreferred. Accordingly, the polyamide resin film of the presentinvention is preferably a stretched film (in particular, biaxiallystretched film). This ensures obtaining each of the above propertiesmore reliably.

Accordingly, the polyamide resin film of the present invention may beconstituted of a single polyamide resin film obtained by polymerizing aregenerated monomer, or may be a mixture of two or more resins, or maybe a polyamide resin film containing a remelted resin as mentionedlater. Furthermore, the polyamide resin film may be constituted of asingle layer, or a film may be constituted of a plurality of layersformed by simultaneous melt extrusion or lamination. For example, thepolyamide resin film may have any of a two type two-layer constitution,in which a polyamide resin film containing a polyamide resin obtained bypolymerizing a regenerated monomer and a polyamide resin film containinga remelted resin, a two-type three-layer constitution, in whichpolyamide resin films containing a polyamide resin film obtained bypolymerizing a regenerated monomer sandwich a polyamide resin filmcontaining a remelted resin, and the like. If the film is composed of aplurality of layers, at least one outermost layer preferably contains apolyamide resin obtained by polymerizing a regenerated monomer.

(3) Use of Polyamide Resin Film

The polyamide resin film (especially a biaxially-stretched polyamideresin film) obtained as above has excellent transparency, color tone,wettability, suitability for printing, and the like, in addition toexcellent mechanical properties such as excellent tensile elongation,tensile strength, and elastic modulus, and therefore, can be usedparticularly as a packaging material.

The polyamide resin film of the present invention may be used singly orin the form of a laminated body laminated with another layer. Forexample, an easy-adhesive layer, a barrier coat layer, a printing layer,an adhesive laver, and the like can be disposed as appropriate.

The polyamide resin film of the present invention may be used as apackaging bag (pouch body) by laminating the film with a sealant layermade from polyolefin or the like to form a laminated film by a knownmethod such as a dry lamination method or an extrusion laminationmethod, and heat-sealing opposing the sealant layers of the laminatefilms. For example, any form such as a two-sided bag, a three-sided bag,a butt-seam bag, a gazette bag, a side seal bag, a standing bag, or astanding zipper bag can be employed as the bag.

Also, the object to be packed (contents) is not limited, and thepackaging bag can be used as a packaging material for a wide range ofproducts, including food and beverages, pharmaceuticals, cosmeticschemicals, and sundries, for example.

The present invention encompasses a laminated film including thepolyamide resin film of the present invention and a sealant resin layerlaminated on the polyamide resin film. Such a laminated film may besuitably used as a packaging material or a package. The polyamide resinfilm of the present invention has a uniform surface state, exhibitslittle local precipitation of additives, monomers, and the like, andcauses no local decrease in adhesion, and therefore, the resultingpackage obtained shows excellent bag breakage resistance. For example, apackage filled with water may be torn from a weak portion when the bagis dropped, and additionally, the bag may be delaminated and torn from aportion where the local adhesiveness is low. In contrast, the laminatedfilm of the present invention has an advantage a bag is harder to breakthan ready-made products.

More specifically, the laminated film of the present inventionpreferably shows a dropping number up to bag breaking of 50 times ormore, preferably 60 times or more, and further preferably 70 times ormore in a bag drop test under an atmosphere at 25° C. and 55% RH. Thebag drop test is carried out by dropping a package (heat-sealed with awidth of 10 mm using two laminated bodies with a size of 200 mm×300 mm),filled with 1000 ml of water and 10 ml of air, from the height. of 1.2m.

EXAMPLES

The characteristic of the present invention will be further specificallydescribed while showing Examples and Comparative Examples. However, thescope of the present. invention is not limited to these examples.

1. Raw Material Used

The raw materials used in Examples and Comparative Examples are asfollows. Table 1 shows the time required for polymerization reaction andthe total treatment time of refining in the preparation or each rawmaterial, as well as the relative viscosities (ηR), crystallizationtemperatures during cooling (Tc) and the half widths thereof, andmonomer content (caprolactam content) of resulting raw materials.

Raw Materials B2 to B4

Film scraps or defective products generated during the production of apolyamide 6 resin film, and resin scraps (resin waste) containingoligomers produced during polymerization of a polyamide 6 resin wereused as the raw material for depolymerization. Phosphoric acid was addedto a raw material for depolymerization, then a depolymerization reactionwas conducted by a wet method under heating, and the reaction mixturewas purified through activated carbon treatment, concentration, anddistillation to recover regenerated ε-caprolactam “C-CL”. On the otherhand, “V-CL” is ε-caprolactam which is a virgin monomer.

C-CL and V-CL were blended so as to be the ratios (mass ratios) shown inTable 1, then the blended mixture was mixed with water and benzoic acidas a terminal blocking agent, and the mixture was heated, pressurized,depressurized, and dehydrated in a polymerization vessel, and afterthat, the polymerization reaction was promoted until the viscosityreached the target viscosity. Required polymerization time for obtainingthe final target viscosity varies depending on the proportion of the CLspecies. Each polymerization time was listed in Table 1. The resultingpolymerization product (resin) was pelletized, then refined by hot watertreatment at 95° C. twice in total for 10 hours and for another 15hours, and dried at 110° C. for 20 hours. Polyamide resins (rawmaterials B2 to P4) with a relative viscosity of 3.1 were obtained inthis way.

Raw Materials B1 and B5

Polymerization was conducted in the same manner as above using C-CL,water, and benzoic acid as a terminal blocking agent, then pelletized,refined by hot water treatment at 95° C. twice in total for 10 hours andfor another 15 hours, and dried at 110° C. for 20 hours polyamide resin(B1) with a relative viscosity of 3.0 was obtained in this way.

Furthermore, polymerization was conducted in same condition as polyamideresin (B1) , the polymerized product was then pelletized, refined by hotwater treatment at 95° C. for 18 hours, and dried at 110° C. for 20hours. A polyamide resin (raw material B5) with a relative viscosity of2.7 was obtained in this way.

Raw Material B6

Polymerization was conducted in the same condition as the raw materialB1, the polymerized product was then pelletized, refined by hot watertreatment at 95° C. for 10 hours, and dried at 110° C. for 20 hours. Apolyamide resin (raw material B6) with a relative viscosity of 2.4 wasobtained in this way.

Raw Material C

Polymerization was conducted in the same step using V-CL, water, andbenzoic acid as a terminal blocking agent, then pelletized, refined byhot water treatment at 95° C. twice in total for 10 hours and foranother 10 hours, and dried at 110° C. for 20 hours. A polyamide resin(raw material C) with a relative viscosity of 3.1 was obtained in thisway.

Raw Material D

Film wastes generated during the production polyamide 6 resin film waspulverized, then re-melted at 250° C. to 290° C., and pelletized. Afterthat, the resulting pellets were dried. A remelted resin (raw materialD) was obtained in this way. The relative viscosity of the remeltedresin was 2.9.

Silica Master F (Silica-Containing Polyamide Resin, Containing 6 mass %Silica)

For producing a master chip, PCM-30, manufactured by IKEGAI CORP., wascharged. with A1030 BRE (a polyamide 6 resin, manufactured by UnitikaLtd.)/silica 1 (SILYSIA 310P, average particle diameter: 1.4 μm,manufactured by Fuji Silysia Chemical Ltd.)/silica 2 (MIZUKASIL P/73,average particle diameter: 2.5 μm, manufactured by Mizusawa IndustrialChemicals, Ltd.) in the ratio of 94.0/4.5/1.5 (mass ratio) from a supplyport, and the content was melt-kneaded at a condition of a cylindertemperature of 230° C. to 270° C., a screw rotation number of 200 rpm,and a discharge amount of 10 kg/hour. The resulting melt-kneaded productwas pelletized and vacuum dried (80° C. for 24 hours). A silica master Fwas obtained in this way. The relative viscosity of the silica masterwas 3.0.

TABLE 1 Crystallization temp. Total Relative Caprolactam during coolingResin Polymerization refining viscosity content Tc Half width (mass %)time (h) time (h) η R (mass %) (° C.) (° C.) B1 C-CL resin 100 16.0 25.03.0 0.12 172.2 10 B2 C-CL/V-CL 20/80 14.0 25.0 3.1 0.15 172.5 13 B3C-CL/V-CL 40/60 15.0 25.0 3.1 0.14 171.7 14 B4 C-CL/V-CL 90/10 15.7 25.03.1 0.13 171.9 11 B5 C-CL resin 100 16.0 18.0 2.7 0.19 172.0 11 B6 C-CLresin 100 16.0 10.0 2.4 0.24 172.0 11 C V-CL resin 100 13.5 20.0 3.10.19 173.0  9 D Remelted resin 100 — — 2.9 0.29 185.7  4

2. Examples and Comparative Examples

Films are prepared using the raw materials listed in section 1.mentioned above.

Example 1

As the composition of a polyamide resin film, 97.5 mass % of a polyamideresin obtained by polymerizing only C-CL (raw material El) and 2.5 mass% of silica master were mixed, then melt-kneaded in an extruder,supplied to a T-die and discharged in a sheet form, and the resultingsheet was wound around a metal drum that was temperature-controlled to20° C., and then cooled and rolled up to obtain an unstretched film witha thickness of about 150 μm. Next, the resulting unstretched film wasimmersed in a moisture control tank, which was controlled to 50° C., for1 minute, then the edges of the unstretched film were held with clips ofa tenter-type simultaneous biaxial stretching device, and theunstretched film was subjected to simultaneous biaxial stretching undera condition at 180° C. and draw ratios in the MD direction of 3.0 timesand in the ID direction of 3.3 times. Thereafter, the stretched film wassubjected to heat treatment at 201° C. for four seconds at a relaxationratio in the ID direct on of 5%, then gradually cooled to roomtemperature, and corona discharge treatment was conducted on one surfaceof the film. After that, the film was rolled to obtain a biaxiallystretched polyamide resin film with a thickness of 15 μm. The totalregeneration ratio in this film was 97.5 mass %. The moisture content ofthe unstretched film was 5 mass %.

Example 2

A biaxially stretched polyamide resin film with a thickness of 15 μm wasobtained in the same manner as in Example 1, except that 97.5 mass % ofa polyamide resin (raw material B5) indicated in Table 1 was used as thecomposition of the polyamide resin film. The total regeneration ratio inthis film was 97.5 mass %.

Examples 3, 5, and 13

Biaxially stretched polyamide resin films with a thickness of 15 pm wereobtained in the same manner as in Example 1, except that 97.5% of apolyamide resin (raw material B4) or (raw material B3) or (raw materialB2) in Table 1 was used as indicated in Table 2 as the composition ofthe polyamide resin film. The total regeneration ratios in these filmswere 87.8 mass %, 39.0 mass %, and 19.5 mass %, respectively.

Examples 4, 11, and 18

Biaxially stretched polyamide resin films with a thickness of 15 pm wereobtained in the same manner as in Example 1, except that a polyamideresin (raw material B1) and a polyamide resin (raw material C) in Table1 were blended according to the proportions as indicated in Table 2, and97.5% of these blended resins were used as the composition of thepolyamide resin film.

Examples 9, 10, 16, and 17

Biaxially stretched polyamide resin films with a thickness of 15 μm wereobtained in the same manner as in Example 1, except that a polyamideresin (raw material B1), a polyamide resin (raw material C), and aremelted resin (raw material D) in Table 1 were blended according to theproportions as indicated in Table 2, and 97.5 mass % of these blendedresins were used as the composition of the polyamide resin film.

Example 8

A biaxially stretched polyamide resin film with a thickness of 15 pm wasobtained in the same manner as in Example 1, except that a polyamideresin (raw material B3) and a remelted resin (raw material D) in Table 1were blended according to the proportion as indicated in. Table and 97 5mass % of these blended resins were used as the composition of thepolyamide resin film.

Examples 14 and 15

Biaxially stretched polyamide resin films with thicknesses of 15 μm wereobtained in the same manner as in Example except that a polyamide resin(raw material B2) and a remelted resin (raw material D) in Table 1 wereblended according to the proportions as indicated in Table 2, and 97.5mass % of these blended resins were used as the composition of thepolyamide resin film.

Example 12

A polyamide resin (raw material B1) and a polyamide resin (raw materialC) in Table 1 were blended according to the proportions as indicated inTable 2 as the composition of the polyamide resin film, then 97.5 massof these blended resins and 2.5 mass % of a silica master were mixed,and an unstretched film was obtained in the same manner as in.Example 1. Next, the unstretched film was stretched in the MD directionat 2.8 times with a temperature-controlled roll at 55° C., the resultingfilm was held with clips, and the film was then stretched in the TDdirection at 3.7 times under the condition at 185° C. (sequentialbiaxial stretching). Thereafter, the stretched film was subjected toheat treatment at 201° C. for four seconds at a relaxation ratio in theTD direction of 5%, then gradually cooled to room temperature, andcorona discharge treatment was conducted on one surface of the film.After that, the film was rolled up to obtain a biaxially stretchedpolyamide resin film with a thickness of 15 μm. The total regenerationratio in this film was 19.5 mass %. The moisture content of theunstretched film was 0.5 mass %.

Example 19

A two-type three-layer constitution film at a thickness ratio of 1:1:1was formed as a polyamide resin film. As an intermediate layer, a resinof a mixture of 25 mass % of a polyamide resin (raw material C) and 75mass % of a remelted resin (raw material C) in Table 1 was disposed. Thepolyamide resin (B3) in Table 1 was disposed as both outer layers. Anunstretched film was obtained in the same manner as in Example 1, exceptthat 2.5 mass % of a silica master was mixed in each layer andco-extruded. The resulting unstretched film was subjected tosimultaneous biaxial stretching to obtain a biaxially stretchedpolyamide resin film with a thickness of 15 μm. The total regenerationratio in this film was 50.4 mass %.

Comparative Example 1

A biaxially stretched polyamide resin film with a thickness of 15 μm wasobtained in the same manner as in Example 1, except that 97.5 mass % ofa polyamide resin (B6) in Table 1 was used as the composition of thepolyamide resin film.

Comparative Example 2

A biaxially stretched polyamide resin film with a thickness of 15 μm wasobtained in the same manner as in Example 1, except that 97.5 mass % ofa remelted resin (D) was used as the composition of the polyamide resinfilm.

Comparative Example

A biaxially stretched polyamide resin film with a thickness of 15 μm wasobtained in the same manner as in Example 12, except that 97 mass % of aremelted resin (D) was used as the composition of the polyamide resinfilm.

Comparative Examples 4 and 5

Biaxially stretched polyamide resin films with thicknesses of 15 μm wereobtained the same manner as in Example 1, except that a polyamide resin(C) and a remelted resin (B) were blended according to the proportionsas indicated in Table 2, and 97.5 mass % of these blended resins wereused as the composition of the polyamide resin film.

Comparative Example 6

A biaxially stretched polyamide resin film with a thickness of 15 μm wasobtained in the same manner as in Example 1, except that 97.5 mass % ofa polyamide resin (C) was used as the composition of the polyamide resinfilm.

Comparative Example

As the composition of a polyamide resin film, 97.5 mass % of a polyamideresin (C) and 2.5 mass % of a master chip (E) were mixed, thenmelt-kneaded in an extruder, supplied to a T-die and discharged n sheetform, and the resulting sheet was wound around a metal drum that hadbeen temperature-controlled to 20° C., and then cooled and rolled up toobtain an unstretched film with a thickness of about 150 μm. At thistime, variations in the actual temperature on the metal drum that hadbeen temperature-controlled to 20° C. were observed, and unevenness ofcrystallinity of the unstretched film were also observed. A biaxiallystretched polyamide resin film with a thickness of 15 μm was obtained inthe same manner as in Example 1 except for the above matters. Portionswhere the thickness accuracy exceeded plus or minus 15% were observed inthe resulting film.

TABLE 2 Raw material (other than E) proportion in film monomer remeltedTotal proportion resin D Silica regeneration Raw Material (mass %)content master E ratio in film Stretching (other than E) C-CL V-GL (mass%) (mass %) (mass %) method etc. Ex. 1 B1 100 0 0 2.5 97.5 SimultaneousEx. 2 B5 100 0 0 2.5 97.5 Simultaneous Ex. 3 B4 90 10 0 2.5 87.8Simultaneous Ex. 4 B1/C 40 60 0 2.5 39.0 Simultaneous Ex. 5 B3 40 60 02.5 39.0 Simultaneous Ex. 6 B1/C/D 30 40 30 2.5 58.5 Simultaneous Ex. 7B1/C/D 30 60 10 2.5 39.0 Simultaneous Ex. 8 B3/D 28 42 30 2.5 56.6Simultaneous Ex. 9 B1/C/D 20 40 40 2.5 58.5 Simultaneous Ex. 10 B1/C/D20 60 20 2.5 39.0 Simultaneous Ex. 11 B1/C 20 80 0 2.5 19.5 SimultaneousEx. 12 B1/C 20 80 0 2.5 19.5 Sequential Ex. 13 B2 20 80 0 2.5 19.5Simultaneous Ex. 14 B2/D 16 64 20 2.5 35.1 Simultaneous Ex. 15 B2/D 1456 30 2.5 42.9 Simultaneous Ex. 16 B1/C/D 10 60 30 2.5 39.0 SimultaneousEx. 17 B1/C/D 10 80 10 2.5 19.5 Simultaneous Ex. 18 B1/C 5 95 0 2.5 4.9Simultaneous Ex. 19 (Outer layer) B3 26.7 48.3 25 2.5 50.4 Simultaneous(Inner) D75% C25% (Multilayer 1/1/1) Com. Ex. 1 B6 100 0 0 2.5 97.5Simultaneous Com. Ex. 2 D 0 0 100 2.5 97.5 Simultaneous Com. Ex. 3 D 0 0100 2.5 97.5 Sequential Com. Ex. 4 C/D 0 50 50 2.5 48.8 SimultaneousCom. Ex. 5 C/D 0 70 30 2.5 29.3 Simultaneous Com. Ex. 6 C 0 100 0 2.50.0 Simultaneous Com. Ex. 7 C 0 100 0 2.5 0.0 CR temperature accuracyinsufficient

Test Example 1

The following properties were examined on the raw material polyamideresins (raw materials B1 to D) and the films obtained in Examples andComparative Examples. Table 3 shows the results. Samples left under anenvironment at a temperature of 23° C. and a humidity of 50% RH for twohours or longer were used in the measurements, and the properties weremeasured under an atmosphere at a temperature of 23° C. and a humidityof 50%

(1) Relative Viscosity ηR

The raw material polyamide resins (raw materials B1 to D) and the filmsobtained in Examples and Comparative Examples were each used as asample, and the relative viscosity sample solution (liquid temperature:25° C.), prepared by dissolving the sample in 96% sulfuric acid at aconcentration of 1.0 g/dl, was measured using an Ubbelohde viscosimeter.

(2) Moisture Content

An unstretched film after water absorption treatment or just beforestretching was sampled, then put in a weighing bottle and dried, and themoisture content was calculated from the difference in mass before andafter drying.

(3) Caprolactam Monomer Content [Preparation of Measurement Sample]

The raw material polyamide resins (raw materials B1 to D) and the filmsobtained in Examples and Comparative Examples were freeze-pulverized,then 0.5 g of the pulverized material was precisely weighed and put in a10-ml headspace bottle, then 10 ml of ultrapure water was added to thebottle, then the bottle was sealed with a butyl rubber stopper and analuminum cap, and the caprolactam monomers were extracted for two hoursin a boiling water bath (100° C.) This content was cooled and filtratedwith a 0.45-μm disk filter, and the resulting filtrate was used as ameasurement sample.

[Production of Calibration Curve]

Caprolactam, 0.1 g, was dissolved in 100 ml of ultrapure water toprepare a 1000-ppm solution, then this solution. was further diluted toprepare standard solutions at 100, 50, 20, 10, 5, and 2 ppm,respectively, and a calibration curve was created.

[HPLC Condition]

Device: HP 1100 HPLC system, manufactured by Hewlett-Packard Company;column: Waters Pureisil 5 μm C18 120 Å, 4.6 mm×250 mm (40° C.),detector: UV 210 nm, injection amount: 10 μl, flow rate: 0.7 ml/min,elution: elution was conducted for 12 minutes with a methanol/water(volume ratio: 35/75) solution, thereafter the eluent was changed to amethanol/water (volume ratio: 100/0) solution over 3 minutes and elutionwas conducted for another 30 minutes, and thereafter, the eluent waschanged to a methanol/water (volume ratio: 35/75) solution over 5minutes and the elution was conducted for further 20 minutes.

[Calculation Method]

The mass of monomers in a sample was calculated from the monomerconcentration of samples detected in the above conditions, and thevalues obtained by dividing the monomer mass by the mass of themeasurement sample were used as a monomer extracted amount (mass %).

(4) Crystallization Temperatures during Cooling (Tc) and Half Width

Crystallization temperatures during cooling (Tc)were measured using adifferential scanning calorimeter (input compensation type DSC 8000),manufactured by Perkin-Elmer Corp., by weighing 10 mg of the resultingresin, then raising the temperature from room temperature to 260° C. ata temperature rising rate of 10° C./min, keeping the temperature at 260°C. for 10 minutes, and cooling the resin to 100° C. at a cooling rate of10° C./min. In the DSC curve with heat flows (mW) on the ordinate axisand temperatures on the abscissa axis, the temperature at the top of thepeak during cooling was taken as Tc (° C.), and the interval between twopoints at the half intensity of the absolute value of Tc after drawingthe baseline from the high-temperature side was taken as a half width (°C.).

(5) Terminal Amino Group Content

The films obtained in Examples and Comparative Examples were dissolvedin a solvent (phenol/ethanol=4/1 in terms of a volume ratio), then acertain amount of 0.02-N hydrochloric acid was added thereto, and thesolution was back-titrated with a 0.02-N aqueous sodium hydroxidesolution.

(6) Terminal Carboxyl Group Content

The films obtained in Examples and Comparative Examples were dissolvedin benzyl alcohol at 180° C., a phenolphthalein indicator was added tothe resulting solution, and the solution was titrated with a 0.02-Nsolution of potassium hydroxide in ethanol.

(7) Evaluation of Number of Missing Dots in 10% GradationPortion/Evaluation of Extensibility of 40% Gradation Portion uponHalftone Printing

[Printing Step]

Printing ink was prepared by mixing a diluent NKFS102 (manufactured byTOYO INK CO., LTD.) in LIOALPHA R39 Indigo (manufactured by TOY INK CO.,LTD.) and adjusting the viscosity (23° C.) of the ink such that theZahn's Cup factor according to #3 should be 15 seconds. A printing rollfilm was prepared by slitting a film at positions 500 mm. away from thecenter leftward and rightward in the TD direction. Using anhelio-engraving gradation-changed plate for gravure printing, the inkwas applied to film, dried at 50° C. for 10 seconds, and the film wasrolled up to produce a printed film.

This gradation-changed plate is a plate helio-engraved in 175 lines (175halftone dots (d t<) per inch width) with gradations of 10%, 20%, 30%,40%, and 100% in the MD direction in this order. The depth of eachgradation was 2.5 μm, 5.0 μm, 7.5 μm, 10.0 μm, and 32.0 μm,respectively, the printed length in the MD direction for each gradationwas 60 mm, and the printed width in the ID direction was 0.8 m.

The main composition of LIOALPHA R39 Indigo contains 10 mass % of apigment, 15 mass % of a synthetic resin, 2.5 mass % of slica, 30 mass %of ethyl acetate, 15 mass % of isopropyl alcohol, 10 mass % of propylacetate, 10 mass % of propylene glycol monomethyl ether, and 5 mass % ofn-propyl alcohol.

The main composition of Diluent. NKFS 102 mainly contains 50 mass % ofethyl acetate, 35 mass % of propyl acetate, 10 mass % of isopropylalcohol, and 5 mass % of n-propyl alcohol.

[Evaluation on Number of Missing Dots]

The number of missing dots was evaluated by sampling a total of 10 areasfrom the 10% gradation portion on the prepared printed film andmagnified 8 times with a stereo microscope ZEISS SteREO Discovery. V12(manufactured by ZEISS) and the number of missing dots (that is, areasthat had not been able to be printed.) was counted in a region with atotal area of 1000 mm² (47500 dots in total) of 20 mm in the MDdirection and 50 mm in the TD direction. The same evaluation wasperformed at 10 sampled regions, and the value with the highest numberof missing dots among all 10 regions is shown in Table 3.

FIG. 1 indicates the outline of the measurement method. In the printingsection with 10% gradation, the regions for evaluation were set in thearea within the remaining 20 mm portions excluding the upper and lowerends 20 mm in the MD direction, and a region with a length of 50 mm fromthe starting point, which was positioned 100 mm apart from the film edgein the ID direction, area was set as the first region. And a total of 10regions at 80 mm intervals were evaluated. That is, as illustrated inFIG. 1, 10 measurement regions a to j having a size of 20 mm×50 mm wereset to evaluation targets.

The number of missing dots may be 100 dots or fewer for practical use inthe total (47500) dots, but is preferably 80 dots or fewer, furtherpreferably 70 dots or fewer, and among them, most preferably 60 dots orfewer. If the above number exceeds 100 dots, the irregularities can bevisually observed even in low-gradation portions by gazing at the area,and individual differences in the pattern can be felt, which may bejudged to be a printing defect.

[Evaluation on Extensibility]

A 40% gradation portion of the prepared printed film was magnified 20times using a stereo microscope ZEISS SteREO Discovery. V12(manufactured by ZEISS), the shapes of the halftone dots were observed,and the halftone dot diameters were measured at an arbitrary 10 points.The evaluation was made at three levels according to the followingevaluation criteria.

(Evaluation Standard)

⊚. . . No problems were observed in the shape of the halftone dots, andthe ratio between the minimum to the maximum of the halftone dotdiameters was 0.9 or more and 1 or less.◯. . . No problems were observed in the shape of the halftone dots, andthe ratio between the minimum to the maximum of the halftone dotdiameters was 0.85 or more and less than 0.9.

Δ. . . Slight distortions were observed in the shape of the halftonedots, and the ratio between the minimum to the maximum of the halftonedot diameters was 0.75 or more and less than 0.85.

x . . . The shape of the halftone dots was distorted, and irregularitiesin the size of the halftone dots were observed; the ratio between theminimum to the maximum of the halftone dot diameters was less than 0.75.

(8) Impact Strength (J)

Samples were prepared by cutting the resulting polyamide resin film insize with a width of 10 cm and a length of 150 cm at the center of thefilm and positions 50 cm away from the center leftward and rightward inthe TD direction, respectively. A film impact tester, manufactured byToyo Seiki Seisaku-sho, Ltd., was used with a ½-inch hemispherical head,and a sample was fixed by clamping in a jig with a 50 cmφ circular hole,and the impact strength was measured 15 times each. Ratios(minimal/maximum) between the maximum and the minimum of impactstrengths are shown in Table 20 The above ratio is _preferably 0.5 to1.0, more preferably 0.6 to 1.0, and further preferably 0.7 to 1.0. . Itis considered that there is a partial reduction in the impact resistanceof the film when the above ratio is less than 0.5, which is notdesirable from a practical standpoint.

(10) Tensile Elongation (%)

Using a DSS-500 autograph manufactured by Shimadzu Corporation, tensileelongation was measured in accordance with the Japanese IndustrialStandard JIS K 7127. region with a width of 10 mm and a length of 150 mmin the MD direction was cut at the center in the TD direction of theresulting polyamide resin film and used as a sample. The measurement wasconducted under a condition of a measurement length of 100 mm (distancebetween holders) and the tensile speed was 500 mm/min, and the tensileelongation was determined by the following expression.

Tensile elongation (%)=Distance between holders at break (mm)/originaldistance between holders (100 mm)×100

(11) Haze (%)

Using a haze meter (NDH 4000) manufactured by Nippon Denshoku IndustriesCo., Ltd., the haze of the center the film in the TD direction wasmeasured in accordance with the Japanese Industrial Standard JIS K 7136.

(12) Wetting Tension (mN/m)

A corona-treated surface of a film was measured using a liquid mixturefor wetting tension tests, No. 36.0 to 54.0 (manufactured by FUJIFILMWako Pure Chemical Corp.) in accordance with Japanese IndustrialStandard JIS K 6768. Regarding the measurement locations, the wettingtension was measured at 5 locations at 200 mm intervals from the centertoward both ends in the ID direction. This was measured every 1 m in theMD direction, and a total of 50 locations were measured. The minimum andthe maximum of values of wetting tensions measured at 50 locations areshown in Table 3. A wetting tension of 44 mN/m or higher is usuallypractical, but in particular 46 mN/m or higher is preferred.

(13) Bag Breakage Resistance (Bag Drop Test) [Lamination Step]

A urethane adhesive (DICDRY LX-401A/SP-60, manufactured by DIC Corp.)was applied to a film surface such that the dry application amountshould be 3.0 g/m², and then heat-treated at 80° C. Then, on theadhesive side after heat treatment, an unstretched polyethylene film(T.U.X MC-S, 50 μm, manufactured by Mitsui Chemicals Tohcello, Inc.) wasdry-laminated on a metal roll heated to 80° C. at a nip pressure of 490kPa. Further aging according to recommendations for the adhesive wasperformed to obtain a laminated film.

[Preparation of Bag Sample]

The produced laminated film was cut into two pieces having a size of 200mm×300 mm. Then, the polyethylene films were put together and heatsealed on three sides with 10 mm widths to form a three-sided bag. Thesealing condition was set to 160° C. for 1 second. A three-way bag wasfilled with 1000 ml of water, then the air inside the bag was released,and the remaining one side was heat-sealed at a width of 10 mm. Then, 10of air was injected into the bag using a syringe, and the bag wastightly sealed again by heat-sealing to prepare a test sample. Thesealing condition was set to 160° C. for 1 second. Adding a small amountof air causes more damage from water against the bag, and the test isconducted under harsher conditions than with water only

[Bag Drop Test]

The prepared test sample was subjected to a test A, in which the lowerend of the test sample was dropped from a height of 1.2 m above a smoothstainless plate (SUS plate) with a thickness of 0.5 mm so that one filmsurface of the test sample hit the SUS plate, and subsequently, a testB, in which the sample was dropped from the same height so that oneshort side of the test sample hit the SUS plate. The tests A and B werealternately repeated until the test sample bag was broken, and thenumber of drops in Test A or B until the bag breakage was counted.

It should be noted that the test sample had two film surfaces and twoshort sides and was dropped so that the same film surface or the sameshort side hit the SUS plate, respectively.

The test was performed under an atmosphere of 23° C. and 50% RH with asample number n=3. Table 3 shows the minimum number of drops before bagbreakage. The number of times up to the bag breakage under theseconditions is required to be 50 times or more for practical use, and 60times or more is particularly preferred, and among them, 70 times ormore is more preferred.

TABLE 3 Bag Terminal Drop Film Capralctam content Missing Extensi-Number MD Wetting relative Extracted (mmol/kg) dot bility ImpactStrength of Elon- tension viscosity amount Amino Caboxyl number in 40%min/ max breakage gation (mN/m) Haze η R (mass %) group group (dots)gradation max min (J) (J) (times) (%) min max (%) Ex. 1 3.0 0.01 47 5491 ⊚ 0.77 0.66 0.86 72 90 50 52 4.2 Ex. 2 2.6 0.03 52 59 87 ⊚ 0.72 0.510.71 68 82 50 52 4.2 Ex. 3 3.0 0.01 51 56 78 ⊚ 0.74 0.67 0.91 80 91 5052 4.1 Ex. 4 3.0 0.01 43 50 62 ⊚ 0.78 0.68 0.87 94 90 50 52 4.3 Ex. 53.0 0.01 38 52 48 ⊚ 0.73 0.67 0.92 107 92 50 52 4.2 Ex. 6 3.0 0.04 64 7066 ⊚ 0.63 0.52 0.83 85 70 50 52 6.7 Ex. 7 3.0 0.02 51 60 62 ⊚ 0.70 0.630.9 76 84 50 52 4.9 Ex. 8 3.0 0.03 69 70 55 ⊚ 0.64 0.54 0.85 98 71 50 526.8 Ex. 9 2.9 0.05 73 74 71 ◯ 0.59 0.49 0.83 65 68 50 52 7.5 Ex. 10 3.00.02 50 67 64 ⊚ 0.67 0.61 0.91 82 80 50 52 5.1 Ex. 11 3.0 0.01 41 49 68⊚ 0.72 0.67 0.93 88 89 50 52 3.8 Ex. 12 3.0 0.11 39 53 94 ⊚ 0.86 0.961.12 59 147 48 52 4.0 Ex. 13 3.0 0.01 45 47 38 ⊚ 0.76 0.71 0.94 106 9050 52 4.1 Ex. 14 3.0 0.02 52 68 43 ⊚ 0.67 0.6 0.9 101 80 50 52 5.0 Ex.15 3.0 0.04 69 70 50 ⊚ 0.62 0.53 0.86 93 71 50 52 6.7 Ex. 16 3.0 0.04 7071 69 ⊚ 0.63 0.55 0.87 81 72 50 52 6.8 Ex. 17 3.0 0.02 53 62 78 ⊚ 0.700.64 0.92 73 83 50 52 4.9 Ex. 18 3.0 0.01 47 48 95 ⊚ 0.74 0.67 0.91 7590 50 52 3.8 Ex. 19 2.9 0.06 75 78 47 ⊚ 0.59 0.34 0.58 77 67 50 52 9.8Com. 2.2 0.03 48 52 — — — 0.10 — 70 48 52 4.3 Ex. 1 Com. 2.7 0.10 108117 200< ◯ 0.48 0.34 0.71 41 42 50 52 9.2 Ex. 2 Com. 2.7 1.80 108 122200< Δ 0.43 0.39 0.91 43 58 42 48 9.4 Ex. 3 Com. 2.8 0.07 80 82 160   ◯0.49 0.43 0.88 48 60 46 50 8.2 Ex. 4 Com. 3.0 0.04 70 72 117   ⊚ 0.610.53 0.87 87 73 50 52 6.7 Ex. 5 Com. 3.0 0.01 43 48 103   ⊚ 0.74 0.680.92 71 93 50 52 4.3 Ex. 6 Com. 3.0 0.01 45 49 165   ◯ 0.59 0.51 0.87 4971 50 52 5.5 Ex. 7

As is apparent from these results, the biaxially-stretched polyamideresin films of Examples 1 to 19 are polyamide resin films made from apolyamide resin obtained by polymerizing recycled monomers, and thushave excellent suitability for printing in low-gradation portions andimpact strength with inhibited unevenness and show no problems ingeneral properties such as film elongation, wettability, and haze, andare excellent in practical performances, as shown in the drop bag test.

In Comparative Example 1, the relative viscosity of the raw materialsused was low, and the mechanical properties of the resulting film werelow. In particular, the impact strength was low. This resulted incutting trouble in the next step, and printing was not able to beperformed normally.

Comparative Examples 2, 4, and 5 do not contain any polyamide resinobtained by polymerizing recycled monomers, and the amount of terminalamino groups or carboxyl groups derived from the remelted resin waslarge, and the crystallization rate of the film material was fast. Thus,local unevenness in the crystallization state of the surface wasobserved, and as a result, the printability of low-gradation portionswas decreased, leading to a decrease in local adhesion in the drop bagtest, which. was thought to cause the bags to break more easily. Also, adecrease in elongation and variation in impact strength that may havebeen caused by impurities in the remelted resin were observed.

The same raw materials as in Comparative Example 2 were used, andsequential stretching was performed in Comparative Example 3. As aresult, the amount of monomers in the film increased. This resulted in alocalized deterioration of suitability for printing in low-gradationportions and similarly resulted in deteriorated results of the drop bagtest. There was also a decrease in elongation and variation in impactstrength that could be attributed to impurities in the remelted resin.

Comparative Example 6 is a film obtained using a polyamide resinconsisting only or a virgin monomer. Although there were no problems ingeneral properties or practical performance, the reproducibility waspoorer than that of a film prepared using a recycled monomer in terms ofprintability of low-gradation portions.

A polyamide resin consisting only of virgin monomer was used inComparative Example 7, but the temperature control of the casting rollwas insufficient, and variations in the degree of crystallinity on thesurface of the unstretched film occurred. Therefore, lacking ofuniformity in the degree of crystallinity on the film surface of thesurface after stretching also occurred, and, as a result, theprintability of low-gradation portions decreased. Furthermore, portionswhere the thickness accuracy after stretching exceeded 15% wereobserved, causing unevenness of impact strength and elongation.

1. A method for producing a polyamide resin film, the method comprising:(1) a step of producing a monomer from a raw material (A) fordepolymerization; (2) a step of producing a polyamide resin (B) throughpolymerization using a raw material con a the monomer; a step ofrefining the polyamide resin (B); and a step of producing an unstretchedfilm using a starting material containing the refined polyamide resin(B), and stretching the unstretched film.
 2. The production methodaccording to claim 1, wherein the monomer contains ε-caprolactam.
 3. Theproduction method according to claim 1, further comprising a step ofadjusting a moisture content in an unstretched film to 2 to 10 mass % inadvance, prior to stretching.
 4. The production method according toclaim 1, wherein starting materials partly contain a remelted resinobtained by melting a polyamide resin (provided that the polyamide resin(B) is excluded) and a content of the remelted resin is 1 mass % ormore.
 5. The production method according to claim 1, wherein the refinedpolyamide resin (B) has a relative viscosity ηR of 2.5 to 4.5.
 6. Theproduction method according to claim 1, wherein the raw material (A) fordepolymerization is at least one of a polyamide 6 resin and an oligomerthereof. 7-15. (canceled)
 16. (ne de resin film obtained by the methodaccording to claim
 1. 17. A polyamide resin film obtained by the methodaccording to claim
 2. 18. A polyamide resin film obtained by the methodaccording claim
 3. 19. A polyamide resin film obtained by the methodaccording to claim
 4. 21. A polyamide resin film obtained by the methodaccording to claim
 5. 21. A polyamide resin film obtained by the methodaccording to claim 6.